At \(\mbox{\textit {Fr}}=2\) , both disk and spheroid wakes transition to the NEQ regime at \(Nt\approx \pi \) . However the transition to the Q2D regime - with enhanced wake decay - is very different; whereas the spheroid wake transitions at \(Nt\approx 15\) , the disk wake does not access the Q2D regime in a domain that spans \(Nt\approx 60\) . Other bluff bodies, e.g., the towed sphere [1]}, [2]} show transition to the Q2D regime at \(Nt \approx 50\) , also delayed with respect to the spheroid wake. The early transition to the Q2D regime of the spheroid wake is driven by its strong modulation – horizontal contraction and expansion of the wake width– in response to the vertical contraction and expansion by the lee waves. This modulation has a particularly strong effect on the slender wake of a spheroid where the horizontal contraction is
a large fraction of the wake width. The early start of the Q2D regime in the spheroid wake is accompanied by a sustained increase of turbulent kinetic energy (TKE), driven by an increase of the horizontal mean shear which acts on the turbulence of the separated boundary layer. The TKE increase is limited to the horizontal velocity with the spanwise component being strongest, having almost an order of magnitude larger energy than the vertical. Although coherent vortical structures and spanwise flapping are seen in the horizontal motion, pancake eddies are not seen until the end of the domain at \(x/D = 80\) .
